Engine starting control has a significant impact on engine emissions and engine speed run-up. Spark placement, relative to piston position, influences both torque and emissions. Torque is necessary to accelerate an engine from cranking speed up to idle speed. Further, low starting emissions are desirable when catalysts are cold and their efficiency is low. In general, advancing spark increases engine torque while retarding spark reduces emissions. Therefore, it is important to provide consistent well-placed spark timing to ensure engine speed run-up with reduced emissions.
One method to adjust spark while an engine is cold is described in U.S. Pat. No. 6,135,087. This method provides spark advance based on coolant temperature and engine speed. Further, the amount of spark advance accounts for engine position and time from the start-to-run transfer. More particularly, the method initially determines whether the desired spark advance is before top dead center and whether the throttle is open. If so, the method uses engine speed and coolant temperature to determine a spark advance multiplier. Thereafter, the current engine position pulse is loaded and an engine position multiplier is interpolated and applied to the spark advance multiplier value. Next, the time since the start-to-run is loaded and a start-to-run multiplier is interpolated and applied to the spark advance multiplier value. Finally, the spark is advanced via the spark advance multiplier value as adjusted by the engine position pulse multiplier and the time since start-to-run transfer multiplier. Upon engine operation reaching an after top dead center condition or when the throttle is closed, the method is exited and the engine is returned to normal spark control.
The inventors herein have recognized several disadvantages of this approach. Namely, the approach changes spark advance based on engine position, thereby resulting in inconsistent spark placement during the engine start. In other words, when an engine is turned off, it generally stops at a random position. In general, key-off removes power from the engine controller and sensors so that engine position data is lost. Consequently, the engine controller monitors several signals during a start to reestablish engine position. Thus, engine position is changing while the engine controller monitors cam and crank signals, attempting to determine engine position. The number of cylinder events before engine position can be established will vary from start to start depending on where the engine has stopped and on the complexity of the engine position monitoring system.
Therefore, if spark based on position is delivered without regard to synchronization between the engine controller and the engine, or without regard to fuel delivery, the angle at which spark is delivered may vary from start to start.
As an example, a fueled cylinder receiving spark may receive spark at an angle intended for the next or prior fueled cylinder. As such, engine position based spark as presented in the prior art, may deliver less than optimal spark.
Furthermore, the method functions only when base spark advance is after top dead center (ATDC) and if the throttle is open. Therefore, the above-mentioned approach does not optimally deliver spark during start where the throttle is closed and retarded spark is used to lower emissions.
Another method to adjust spark when and engine is cold is described in U.S. Pat. No. 5,483,946 owned by the assignee of the present invention. The method describes retarding ignition timing from a nominal value during a period following engine start and returning the ignition timing to the nominal value by termination of the period, where the period is based on time.
The inventors herein have also recognized that while this approach works well during cold engine operation, it can be inaccurate during start because the method adjusts spark in relation to time. Spark based on time delivers a spark advance that is a function of time since the timer is started. However, there is not a one to one relationship between engine position and time due to variability in engine stopping location as discussed above. Further, engine position is a mechanical dimension; time is a continuum, which lacks spatial dimensions.